Accurate high-resolution control voltages are not easy to generate quickly. Generated voltages can be accurate and can have high resolution and can be quickly generated (short settling time) so long as the designer is willing to incur high cost and great circuit complexity. For example, high-cost monolithic digital-to-analog (D/A) integrated circuits are available which provide guaranteed resolution and linearity.
If the designer is wing to tolerate a very long settling time, the generating circuit can be less expensive. For example, a so-called delta-sigma architecture may be used, which is basically a pulse width modulator followed by a low-pass filter.
Another prior-art approach, shown in FIG. 1, is to employ two or more D/A convertors 22, 24 with appropriate gain or attenuation blocks 23, 25 prior to a linear summation stage 20 having an output 21. These components taken together are a composite D/A convertor 103. In FIG. 1, D/A convertor 22 is the xe2x80x9ccoarsexe2x80x9d convertor, where each stepwise change in the input control lines 101 makes a large change in the output of the convertor 22. D/A convertor 24 is the xe2x80x9cfinexe2x80x9d convertor, where each stepwise change in the input control lines 102 makes a small change in the output of the convertor 24. The usual design goal is to adjust the gain or attenuation blocks 23, 25 so that the composite D/A convertor 103 comes as close as possible to having a linear transfer characteristic as possible. On a practical level, however, there is finite accuracy of the individual elements, there are temperature tracking problems, etc. As a consequence, it is not possible to create a monotonic composite D/A convertor 103 with constant and equal steps everywhere over the entire output range.
The designers of the systems described above are typically designing for a system where it is desired to be able to generate an arbitrary output voltage level without any knowledge of the previous setting of the D/A convertor. Stated differently, every required output voltage level 21 is generated unambiguously from a single setting of the digital control lines 101, 102. Such systems are particularly helpful in cases lacking feedback, that is, in open-loop applications. As such, however, the systems suitable for such open-loop applications are, as mentioned above, very expensive or very slow to settle or both.
It is desirable, then, to consider whether there are applications which require D/A convertors but where the circumstances (e.g. availability of feedback, absence of any requirement of overall monotonicity) permit completely different approaches to generation of analog control signals based on digital inputs.
A digital-to-analog convertor is described which provides accurate and high-resolution results in particular constrained applications where feedback is available and where there is no need for monotonicity across the entire dynamic range. The convertor comprises a capacitor stack, the common point of which is the output of the convertor. Digitally controlled switches, which may be discrete outputs from a microcontroller, selectively apply first or second potentials to points in the capacitor stack, either directly or through resistors. Appropriate control of the switches permits developing desired output voltages quickly and accurately. Performance equivalent to 24-bit 1 least-significant-bit (LSB) accuracy is easily attainable even with components having 5% tolerance.